In circuits made with CMOS technology, where programmable components are used to make non-volatile memories (ROM, EPROM, EEPROM, FLASH), it is necessary to provide a reference system which is able to recognize programmed elements, which are turned off and which therefore draw a zero current when driven, from the nonprogrammed, or virgin, elements, which are on, and therefore draw a current when driven. Recognition is achieved by comparing, in a differential sense amplifier, the current in the element or cell which it is desired to read with the current in a reference cell, which may itself be nonprogrammed or programmed.
However, the sense amplifier must be able to provide an output also when reading a nonprogrammed matrix cell, in which case both its inputs would be equal. Therefore, it is known to shift the discriminating threshold by a method known as "variable offset current", according to which an offset current of half the current in a nonprogrammed cell is permanently applied to the reading input of the amplifier.
While the above approach gives satisfactory results in a limited range of supply voltages, it is liable to fail at high supply voltages, because (as will be shown in more detail below) then the current in the programmed cell increasingly departs from zero, until it exceeds the value of the threshold current (i.e., half the current in a nonprogrammed cell).
The above situation effectively limits the range of acceptable supply voltages, or, from another point of view, compels the manufacturer to discard devices which would otherwise operate satisfactorily at middle voltages, because they fail at the high end of the voltage range.
The main object of the invention is therefore to provide an improved sense circuit, of the variable offset current type, for recognizing the nonprogrammed or programmed status of the cells in storage devices such as non-volatile memories, which improved sense circuit compensates the above drift of the current in the programmed cell, and extends the operating range of the sense circuit, particularly in that it allows operation in a wider range of voltage supplies.
Another object is to increase the manufacturing yield of such sense devices.